1. Field of the Invention
The present invention relates to optical devices which comprise a substrate which supports at least one semiconductive polymer which is less prone to aggregation than conventional poly(fluorene)-based polymers and which exhibits a blue-shifted emission. It also relates to novel homopolymers and copolymers for use in said devices, monomers for the preparation of said polymers and a process for the preparation of statistical copolymers which is particularly suitable for the preparation of the polymers of the present invention.
2. Description of Related Technology
In recent years, there has been considerable interest in conjugated polymers. These are polymers which possess a delocalised pi-electron system along the polymer backbone. The delocalised pi-electron system confers semiconducting properties to the polymer and gives it the ability to support positive and negative charge carriers with high mobilities along the polymer chain. Thin films of these conjugated polymers can be used in the preparation of optical devices such as light-emitting devices. These devices have numerous advantages over devices prepared using conventional semiconducting materials, including the possibility of wide area displays, low dc working voltages and simplicity of manufacture. Devices of this type are described in, for example, WO-A-90/13148, U.S. Pat. No. 5,512,654 and WO-A-95/06400.
The world market for displays based on organic and polymeric light-emitting materials has recently been estimated by Stanford Resources, Inc., to be $200 million in the year 2002 with a strong growth rate which fuels the high industrial interest in this area (D. E. Mentley, “Flat Information Displays: Market and Technology Trends”, 9th edition, 1998). Efficient and highly stable LED devices with low power consumption, which fulfill commercial requirements, have been prepared by a number of companies and academic research groups (see, for example, A. C. Grimsdale et al., Angew. Chem. Int. Ed. 1998, 37, 402; R. H. Friend et al., Nature 1999, 397, 12). As a result of this very fast development of polymer-based LEDs (PLEDs) compared to the development of inorganic LEDs (Sheats et al., Science 1996, 273, 884) the first effective monochromatic active- and passive-addressed matrix displays have been demonstrated to work and in 1999 PHILIPS announced the initiation of a manufacturing line for PLED display components (e.g. LEP backlights for the automotive and telecommunications industries).
At the moment, great efforts are dedicated to the realization of a full-colour, all plastic screen. The major challenges to achieve this goal are: (1) access to conjugated polymers emitting light of the three basic colours red, green and blue; and (2) the conjugated polymers must be easy to process and fabricate into full-colour display structures. PLED devices show great promise in meeting the first requirement, since manipulation of the emission colour can be achieved by changing the chemical structure of the conjugated polymers. However, while modulation of the chemical nature of conjugated polymers is often easy and inexpensive on the lab scale it can be an expensive and complicated process on the industrial scale. The second requirement of the easy processability and build-up of full-colour matrix devices raises the question of how to micro-pattern fine multicolour pixels and how to achieve full-colour emission. Inkjet printing and hybrid inkjet printing technology have recently attracted much interest for the patterning of PLED devices (see, for example, R. F. Service, Science 1998, 279, 1135; Wudl et al., Appl. Phys. Lett. 1998, 73, 2561; J. Bharathan, Y. Yang, Appl. Phys. Lett. 1998, 72, 2660; and T. R. Hebner, C. C. Wu, D. Marcy, M. L. Lu, J. Sturm, Appl. Phys. Lett. 1998, 72, 519).
In order to contribute to the development of a full-colour display, conjugated polymers exhibiting direct colour-tuning, good processability and the potential for inexpensive large-scale fabrication are sought. The step-ladder polymer poly-2,7-fluorene has been the subject of much research into blue-light emitting polymers (see, for example, A. W. Grice, D. D. C. Bradley, M. T. Bernius, M. Inbasekaran, W. W. Wu, and E. P. Woo, Appl. Phys. Lett. 1998, 73, 629; J. S. Kim, R. H. Friend, and F. Cacialli, Appl. Phys. Lett. 1999, 74, 3084; WO-A-00/55927 and M. Bernius et al., Adv. Mater., 2000, 12, No. 23, 1737). This class of conjugated polymers possesses excellent processability, endowed by the attachment of solubilizing groups at the remote C-9 position, without hampering the extended conjugation and therefore leading to high fluorescence quantum yields in the solid state (up to 79%) (see, for example, Q. Pei, Y. Yang, J. Am. Chem. Soc. 1996, 118, 7416). Other benefits of poly-2,7-fluorene are its excellent thermal (Td>400° C.) and chemical stability and its good film forming properties. The rigid nature of this polymer, however, enhances inter-chain aggregation leading to an undesired red-shift of the emission colour and a decreasing luminescence efficiency through excimer formation (see, for example, V. N. Bliznyuk, S. A. Carter, J. C. Scott, G. Klarner, R. D. Miller, and D. C. Miller, Macromolecules, 1999, 32, 361). Aggregation has been decreased to some extent through statistical copolymerization of 2,7-dibromofluorene with other halogenated monomers.
The process to make homo- and copolymers based on 9,9-disubstituted fluorene monomers depends on the metal-mediated cross coupling of both AA-BB and AB type monomers. There is now a considerable prior art in the field. Such copolymers may be made by the cross coupling of dibromo-substituted monomers by contacting them with a Ni(0) catalyst formed in situ from a Ni(II) salt (the Yamamoto coupling, Progress in Polymer Science, Vol. 17, p.1153, 1992) (E. P. Woo et al., U.S. Pat. Nos. 5,708,130; 5,962,631). A Pd(0) mediated cross coupling between arylboronic acids and esters and aryl or vinyl halides (the Suzuki coupling, A. Suzuki et al., Synth. Commun., 1981, 11, 513) has been developed in the presence of a phase transfer catalyst and an inorganic base to make relatively high quality poly(fluorene) derivatives for applications as PLEDs (M. Inbasekaran, U.S. Pat. No. 5,777,070). Extension to various comonomers having hole transporting properties has also been realised (WO-A-99/54385). In a further development a combination of a catalyst and a base was selected to convert the boron functional groups into —BX3− where X is either F or OH (WO-A-00/53656).
As noted above, it is well known that a major disadvantage of poly(fluorene)-based homopolymers is their tendency to aggregate in the solid state, resulting in the formation of excited state complexes (excimers) under conditions of fluorescence through stimulation by photo-excitation or double charge injection (electroluminescence). One way of reducing this tendency is to employ copolymers to break up aggregation (see U.S. Pat. No. 5,777,070; D. Kim, et al., Macromolecules, 1999, 32, 1476). Another approach is to use ladder-like planarised polymers (U. Scherf and K. Müllen, Adv. Polym. Sci., 1995, 123, 1) and poly(indenofluorenes (S. Setayesh et al., Macromolecules, 2000, 33, 2016). Dendrimer substituents as end caps (G. Klaerner, R. D. Miller and C. J. Hawker, Abstracts of Papers of the American Chemical Society, 216: 300-POLY, Part 3 Aug. 23 1998) and at the 9-position of the fluorene building block (S. Setayesh et al., J. Am. Chem. Soc., 2001, 123, 946) have been used to inhibit aggregation. Hole trapping end groups also enhance device performance, possibly through inhibition of aggregate formation.(T. Miteva et al., Adv. Mater., 2001, 13, 565). It has been noted that poly(1,4-phenylene vinylene) homo- and copolymers carrying a 2,3-disubstitution pattern show a tendency to twist, and this distortion has been used to improve luminescence efficiency of polymer derivatives (see WO-A-01/07052). The origin of the broken conjugation in the backbone m-linked polyphenylenes has recently been discussed by S. Y. Hong et al., Macromolecules, 2001, 34, 6474.
It is highly desirable to develop electroluminescent polymers which reduce the aggregation seen in poly(fluorene)-based polymers. In this invention the design of electroluminescent polymers incorporating comonomers based on homologues of fluorene derivatives and optical devices incorporating said polymers is described.